Important Issues in the Design of the FNPSNS Beamline

Geoff Greene

In anticipation of next week's workshop, we wanted to call to your
attention to some important issues regarding the design of the beamline(s)
for fundamental physics at the SNS. We hope that the advanced notice of
these issues will help make the workshop even more productive. The
important questions to be addressed include:

Should we request a dual or single beamline from the SNS? As the
name implies, a dual beamline comprises two beamlines in a single shutter.
Each experiment would have its own neutron guide, choppers, and etc. The
main advantage of this approach is that it can make possible two
independent experiments in slightly more floor space than a single
experiment. The main drawbacks are that there is less room for each
experiment, the flux may be less than on a single beamline, and the
backgrounds could be higher. SNS scientists plan to construct Magnetism and
Liquids Reflectometers on a dual beamline. A good deal of design work has
been done for these beamlines and it is felt that the approach will work
for these instruments despite the fact that their background requirements
are rather stringent (see
http://www.sns.gov/users/instrument_data_sheets.pdf
for a brief description
of the reflectometers and the layouts of other instruments). Hence, it is
likely that a similar design may work for fundamental physics experiments.

Should the neutron guide be extended into the "core vessel insert"?
This is the region between the moderator and the beginning of the main
shutter (1 2.2 m from the moderator). Access to this area is more limited
than for the shutter, and the radiation is higher. However, our simulations
show that the cold neutron flux at the end of a reasonable guide is
significantly lower if the guide starts at 2.2 m instead of 1 m. Designing
a guide for this region is more difficult for the dual beamline, but our
current understanding of the relevant dimensions indicates that it is
possible to extend both guides to within 1 m of the source and not have
them overlap. Drawings of possible dual and single beamline scenarios are
available on the workshop web site
(http://www.phy.ornl.gov/workshops/neutrons/).

What are the optimum types and lengths of the neutron guides?
Presumably, the guides will be curved to reduce backgrounds from
high-energy neutrons and gamma rays, and hence eliminate the need for a
t-zero chopper to perform this task. However, a curved guide would result
in some loss of flux at shorter wavelengths. The experiments appear to
separate into two classes; those requiring maximum flux across a range of
wavelengths, and those requiring 9 A neutrons for UCN production in LHe.
The minimum flight path length is probably on the order of 15 m. The
maximum flight path length to keep the experiment inside the experimental
hall is about 25 m. Should tapered guides be considered for increasing the
flux over a smaller area while increasing the divergence?

How many and what type of choppers are needed? Following the
example of the Reflectometer beamlines, 3 choppers probably will be needed
to define the proper wavelength frame(s). Should we consider other
approaches (e.g. mirror wedges) to eliminate long wavelength neutrons? What
approach should be used to select 9 A neutrons?

In addition to addressing the technical issues noted above the workshop
will also serve to initiate activities leading to the establishment of
fundamental neutron physics "Instrument Development Team" (IDT). A brief
introduction to the IDT process at the SNS can be found at:

Roughly speaking, the IDT concept is based upon a model that has become the
norm at DOE Basic Energy Sciences funded user facilities (especially
synchrotron light sources). Examples of this model are the PRT's
(Participating research Teams) at the NSLS, and the CAT's (Collaborative
Access Teams) at the APS. In the BES world, IDT-like collaborations secure
funding for instruments that are not included in the scope of the initial
facility construction project. To a greater or lesser extent, the IDT is
expected to design a user instrument and to take responsibility for its
construction. In return for their efforts in raising money, and for their
time devoted to the construction of the instrument, the IDT receives a
fraction of the beam time for their own research. The remaining beam time
is distributed to other users through a facility run competitive proposal
process. The fraction of time given to the collaboration, and the
operational expectations for the collaboration vary from facility to
facility. The allocation of beam time is usually for a fixed period of time
(3-5 years) with possibility for renewal following a review. For the SNS,
these details are in the process of being finalized.

It is important to note that the anticipated scope of the IDT's activities
relate to the construction of the beamline(s) and appropriate user support
for experiments that are carried out at the SNS. We anticipate that capital
equipment funding for the construction of individual experiments will be
approved and allocated separately. Of course one anticipates that IDT
members will be heavily involved in these experiments, but that their role
in the development and execution of the experimental program is outside
their role as IDT members.

The character of fundamental neutron physics experiments is rather
different than those carried out at the neutron scattering spectrometers
(either part of the SNS project of built by other IDT's). Our challenge is
to put together an organizational structure that satisfies the strictures
of the SNS project while accommodating the special needs of this type of
research. In doing so, we must bear in mind that our management of this
project must satisfy the needs of several constituencies including the SNS
project, the Physics Division at ORNL, our sponsoring organizations (DOE NP
and NSF Nuclear Physics), The DOE office of Basic Energy Sciences, and, of
course, our user community (in and out of the IDT). In the following, we
list a few of the concerns that should be addressed.

The SNS management recognizes that the character of the Fundamental Neutron
Physics effort will be rather different than that at the other beamlines at
SNS. They appear to be prepared to give us rather broad latitude in our
management of our project as long as they are convinced that we are making
productive use of our beamline. Given that the SNS project does not at the
moment include expertise in our fields of research we will need to work
with the SNS to establish appropriate external panels that can provide
appropriate review of our efforts. In particular, the SNS program advisory
committee will not necessarily include a nuclear physics panel. Both the
SNS management and the funding agency responsible for the SNS construction
and operation (BES) must see a transparent process that ensures the quality
of the science at their facility.

It is anticipated that support for construction and operations of the
fundamental neutron physics beamline will be funded through the Physics
Division at ORNL (with subcontracts to other institutions as appropriate).
The IDT must therefore be responsive to line management at ORNL with
respect to fiscal and project management.

The selection and scheduling process for experiments at the SNS must be
acceptable to the agencies (most likely DOE NP and NSF) that will provide
funds for each experiment. An appropriate level of coordination must be
established to ensure that scarce funding dollars and scarce beam time are
coordinated and used effectively.

Finally, and perhaps most critically, the operation of the beamline must be
carried out in a way that meets the needs of the individuals and teams
contemplating experiments at the SNS. This implies adequate technical
support as well appropriate facilities and amenities at the SNS. It also
implies that there be an open and clearly defined proposal, review, and
scheduling process that allows all users to make productive use of both
their own time and the beam time at the SNS. we should, as well, take
cognizance of the fact that it may prove very difficult to make a
distinction between the IDT and the "outside" user community. Indeed it may
be that this distinction is not appropriate for our beam line.
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